Traumatic brain injury (TBI) remains a major medical problem, resulting in 50,000 deaths/year in the US and is an important cause of long-term morbidity. In the present application we propose to explore the mechanism of action of a tPA mutant (tPA-S481A) that lacks fibrinolytic activity and does not prevent loss of blood-brain-barrier (BBB) permeability exerts a powerful neuroprotective effect. Our data help explain this seemingly paradoxical outcome by showing the beneficial effects of maintaining BBB permeability by 1) accelerating the clearance of neurotoxic amino acids and 2) promoting the delivery of tPA-S481A to brain parenchyma where it competes with endogenous tPA for activation of the NMDA receptor and thereby prevents neuronal apoptosis. These studies provide both fundamental new insights into the function of BBB permeability post-TBI and identify a novel mechanism of neuroprotection. PUBLIC HEALTH RELEVANCE: We will explore the mechanism of action of a tPA mutant that exerts a powerful neuroprotective effect against traumatic brain injury (TBI) in the absence of fibrinolytic activity and without preventing loss blood-brain-barrier integrity. We will explore this seemingly paradoxical outcome by showing how opening of the BBB during brain injury provides neuroprotection by clearing neurotoxic agents and ameliorating apoptosis by preventing NMDA receptor activation.